Not applicable.
Not applicable.
This invention relates to valves used in environments susceptible to the formation of hydrates. More particularly, this invention relates to methods and apparatus for preventing the formation of hydrates in valves, namely gate valves and ball valves.
Clathrate hydrates are crystalline compounds that occur when water forms a cage-like structure around guest molecules, particularly gaseous molecules. Clathrate hydrates, especially in the petroleum industry, are referred to as gas hydrates, gas hydrate crystals, or simply hydrates. Typical hydrates formed in petroleum (hydrocarbon) environments are composed of water and one or more guest molecules such as methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide, and hydrogen sulfate. In general, hydrates will form when a mixture of water and hydrocarbon gases are mixed at high pressures and low temperatures.
The formation of hydrates is of particular concern in subsea hydrocarbon exploration and production where water and gaseous hydrocarbons are often in close proximity at high pressures and low temperatures. If hydrates form within subsea components they are capable of preventing actuation of critical components and of blocking the flow of fluids through the system. It is therefore desirable to take provisions to prevent the formation of hydrates in these systems.
To overcome these problems, several thermodynamic measures are possible in principal: removal of free water, maintaining an elevated temperature and/or reduced pressure, or the addition of freezing point depressants (antifreeze). As a practical matter, the last mentioned measure, i.e., adding freezing point depressants, has been most frequently applied. Thus, lower alcohols and glycols, e.g., methanol, have been added to act as antifreezes. It has been known that in lieu of antifreezes, one can employ a crystal growth inhibitor that inhibits the formation of the hydrate crystals and/or the agglomeration of the hydrate crystallites to large crystalline masses sufficient to cause plugging. Thus, surface active agents such as phosphonates, phosphate esters, phosphonic acids, salts and esters of phosphonic acids, inorganic polyphosphates, salts and esters of inorganic polyphosphates, polyacrylamids, and polyacrylates have been used.
One application that is particularly susceptible to the formation of hydrates is the secondary recovery system known as Water Alternating Gas (WAG). In a WAG system, alternating volumes of water and hydrocarbon gases are injected through an injection well into a hydrocarbon bearing formation in order to force the stored hydrocarbons into production wells drilled in the same formation. This technique is used to increase the volume of production through the adjacent production wells. When used in cold environments, including subsea, the water and the gas are often mixed at high pressures and low temperatures which are often close to the conditions at which hydrates will form.
Hydrates that form in the WAG flowline are a concern but are easily prevented by directly injecting chemicals into the flowline. More difficult is the prevention of hydrate formation within the cavity of valves used to control the flow of water and gas. If hydrates form within the valve cavities, the valves can no longer be opened or closed and the system must be shut down. Simply injecting an inhibiting chemical into the valve cavity has the potential problem of forcing material across the valve seal faces and possibly washing out the seals.
Therefore, there remains in the art a need for methods and apparatus to prevent the creation of hydrates within valve manifolds and in particular within the valve cavities. Therefore, the present invention is directed to methods and apparatus for allowing the injection of chemicals into a valve cavity without risking washout of the valve seals.
Accordingly, there is provided herein methods and apparatus for allowing the injection of hydrate inhibitors into a valve cavity without washing out the valve seals. The present invention generally comprises a valve having a sealing member, such as a gate or a ball, that provides for fluid communication between the valve cavity and the valve flowbore. Fluid communication between the valve cavity and the valve flowbore provides a direct fluid path and prevents a buildup of pressure within the cavity, thus preventing washout of the valve seals.
One embodiment of a valve constructed in accordance with the present invention is an expanded gate valve comprising a valve body having a flowbore intersecting a valve cavity and a gate assembly disposed within said cavity. The gate assembly is a parallel expanding gate assembly having ported, juxtaposed members that are moveable into a sealing arrangement with upstream and downstream valve seats disposed about the flowbore. The gate assembly further comprises a flow path that enables direct fluid communication between the aligned ports and the valve cavity. This flow path enables hydrate inhibitors injected into the valve cavity to flow freely into the port and the flowbore without crossing the sealing faces of the gate assembly.
One embodiment of a valve manifold employing aspects of the present invention comprises a first valve that controls flow from a water inlet and a second valve that controls flow from a gas inlet. Both valves are connected to a common outlet. Each valve comprises a valve body having a flowbore intersecting a valve cavity in which is disposed a sealing member. Each valve is also adapted to receive hydrate inhibitors, such as methanol, injected directly into the valve cavity. Each sealing member has features that, in an open position, allow direct fluid communication between the valve cavity and the flowbore without effecting the performance of the valve through washout or erosion of any sealing surfaces.